Electrochemical fuel cells convert the chemical energy of fuels directly into electrical energy to provide a clean and highly efficient source of electrical energy. Like a battery, a fuel cell consists of two electrodes (an anode and a cathode) separated by an electrolyte typically made of a thin polymeric membrane. In a typical fuel cell, hydrogen gas from the fuel reacts electrochemically at the anode electrode and is converted into protons and electrons. The protons move through the electrolyte to the other electrode, where they combine with the product from the reduction of oxygen from the air at the cathode to form water, which is expelled from the cell as vapor. The involvement of hydrogen and oxygen in the two reactions—one releasing electrons and the other consuming them—yields electrical energy that is tapped across the electrodes for electrical power.
The high conversion efficiencies and low pollution of fuel cells such as hydrogen and direct methanol fuel cells are becoming increasingly attractive power sources for mobile and stationary applications such as on-board electric power for advanced propulsion systems and generation of non-polluting vehicles. While researchers around the world are developing potential fuel cell applications including electric vehicles and portable electrical power supplies, these developments faces challenging scientific problems in the areas of materials science, interfacial science and catalysis. In proton exchange membrane fuel cells (PEMFCs) hydrogen ions must be transported through a semi permeable membrane, hydrocarbon fuels must be converted to pure hydrogen by reforming, and the overall conversion requires a complex process technology and substantial investments in safety and controls. Direct methanol fuel cells (DMFCs) offer a simpler solution and require no reformer.
Direct methanol fuel cells are increasingly considered as an attractive power source for mobile applications because of the high energy density, the fuel portability, and the easily renewable feature of methanol. The fuel portability of methanol is particularly important in comparison with the difficulties of storing and transporting hydrogen. For methanol oxidation, the binary PtRu nanoparticle catalyst on carbon support is currently one of the most-studied catalysts, and shows a bifunctional catalytic mechanism, in which Pt provides the main site for the dehydrogenation of methanol and Ru provides the site for hydroxide (OH) and for oxidizing CO-like species to CO2.
Two technical concerns retard the use of direct methanol fuel cells. First, currently, the energy density (˜2000 Wh/kg) and operating cell voltage (0.4 V) for methanol fuel cells are much lower than the theoretical energy density (˜6000 Wh/kg). Second, the thermodynamic potential (˜1.2 V) due to poor activity of the anode catalysts and “methanol cross-over” to the cathode electrode, leads to a loss of about one-third of the available energy at the cathode and another one-third of the available energy at the anode.
In addition, concerns exist with the use of platinum group metals (PGM) for both anode and cathode catalysts. PGM are quite expensive, and a method of reducing the amount of PGM required in a direct methanol fuel cell will make these cells more commercially attractive. In addition, a major problem with the PGM catalysts is the poisoning of Pt by CO-like intermediate species. On the cathode, the kinetic limitation of the oxygen reduction reaction (ORR) is a problem of interest in proton exchange membrane fuel cells operating at low temperature (<100° C.) and in DMFCs. The rate of breaking O═O bond to form water strongly depends on the degree of its interaction with the adsorption sites of the catalyst, and competition with other species in the electrolyte (e.g., CH3OH). A problem in using Pt as catalyst at the anode is the strong adsorption of OH forming Pt—OH, which causes inhibition of the O2 reduction.
Bimetallic AuPt is a known electro-catalyst for oxygen reduction in alkaline fuel cells. However, there have been few reports for the utilization of AuPt nanoparticles with controllable size and composition in fuel cell catalyst applications. Such a use is important because the metal nanoparticles in the size range of 1 to 10 nm undergo a transition from atomic to metallic properties, and the bimetallic alloy composition produces a synergistic effect. The synergistic catalytic effect involves the suppression of adsorbed poisonous species and the change in electronic band structure to modify the strength of the surface adsorption. With bimetallic Au and Pt systems, Pt functions as main hydrogenation or dehydrogenation sites, and the use of Au together with Pt speeds up the removal of the poisonous CO-like species. Observations of the function of AuPt bulk alloy catalysts include: the decrease of activation energy for facilitating oxidative desorption and suppressing the adsorption of CO; the sufficiently-high adsorptivity to support catalytic oxidation in alkaline electrolytes; the important role of OH−ads in alkaline medium; and the presence of Au playing a role in reducing the strength of the Pt—OH formation. It has been recently shown that catalysts prepared by impregnation from Pt and Au precursors are similar to those of monometallic Pt catalysts, indicating that the presence of Au did not affect the catalytic performance of Pt in any significant way, because the two metals remain segregated due to their miscibility gap, and only Pt participates in the adsorption of CO and the reactions under consideration. In contrast, catalysts prepared from a AuPt organo-bimetallic cluster precursor exhibited different behavior both in terms of CO adsorption and their catalytic activity, suggesting that Pt and Au remain intimately mixed in the form of bimetallic particles and that the presence of Au modifies the catalytic properties of Pt.
The gold and gold-platinum nanoparticles prepared by two-phase protocol are first assembled on carbon black support materials and then activated by calcination, and finally deposited on planar glassy carbon substrates (electrodes). An initial comparison of the electro catalytic ORR activities of carbon-supported Au and AuPt nanoparticle catalysts with commercially-available Pt/C and PtRu/C catalysts is also made. Co-precipitation, deposition-precipitation, ion-exchange, impregnation, and successive reduction and calcination have been widely used for the preparation of the catalysts (3, 4, 15). The lack of control over size, shape and stability is an inherent disadvantage of these previously known methods of preparation. It is especially difficult to process the nanoparticles once produced. Stabilizing the surface of the nanoparticles by capping them with a shell of organic molecules has been used in the current invention, achieving controllable size, shape, composition, and surface properties of the particles.
The present invention provides “core-shell” assembled gold and gold/platinum nanoparticles to fabricate a new class of catalysts. The present invention provides a method of preparation of these AuPt nanoparticles controlling their size, shape, composition and surface properties. As a result, the nanoparticles are both aggregation resistant and resistant to poisoning by CO-like species typically present in DMFCs.